1. Field of the Invention
The present invention relates to a head position controlling method in a magnetic recording reproduction apparatus such as VTR, and more particularly to a head position controlling method wherein high picture quality is obtained in various modes.
2. Description of the Prior Art
In a VTR as shown in FIG. 8, a magnetic tape guide drum 1 has a rotary drum (upper drum) 1a, and two recording reproduction heads (video heads) 2, 3 are installed on the rotary drum 1a and spaced fron each other by 180.degree.. A magnetic tape 4 is traveled in contacting obliquely with cylindrical surface of the magnetic tape guide drum 1, thereby traveling track of each head on the magnetic tape (called "video track") is inclined with respect to the tape longitudinal direction and the traveling track of each head is shifted in sequence by one track pitch. Thus the two heads 2, 3 are changed and used alternately, and signals are recorded to the magnetic tape 4 or read from the magnetic tape 4 and reproduced. In other words, the two heads 2, 3 are scanned alternately as if moved in threadably motion on the surface of the magnetic tape traveling in wound state on the cylindrical surface of the magnetic tape guide drum 1, thereby signals are recorded in sequence to the magnetic tape or read from the magnetic tape. In the magnetic tape guide drum 1, numeral 1b designates a stationary lower drum.
In such VTR, accuracy of the tape guide (lead) of the lower drum 1b, installation accuracy of the video heads (rotary magnetic heads) 2, 3, vertical accuracy of the rotary shaft and the like are controlled strictly, thereby the track format with the track pitch of about 10.about.20 .mu.m is attained and the picture image with assured quality can be displayed on a cathode ray tube of a television.
In recent years, not only ordinary reproduction but also special reproduction such as still reproduction, slow reproduction or speed search must be performed at high picture quality. In other words, variable speed reproduction is required at noiseless state and high picture quality. In order to satisfy such requirement, the reproduction output must be made maximum at various modes such as ordinary reproduction, still reproduction, slow reproduction, speed search or the like.
Consequently, one method is proposed wherein a special reproduction head is newly installed in addition to an ordinary recording reproduction head so that the head constitution is made 3.about.5 heads for example, and the tape feeding is controlled so as to reduce the noise. FIG. 9 is a diagram illustrating such head constitution in the prior art, and FIG. 10 is a correspondence table between modes and used heads. Four heads R, L; R', L' are installed to the rotary drum 1a. According to such method, the noiseless state is attained in combination of heads suitable to various modes, but increase of the heads in number is inevitable.
Another method is proposed wherein special reproduction is performed at high picture quality using a piezoelectric element. FIG. 11 is an exploded perspective view of a rotary head using such piezoelectric element. In FIG. 11, numeral 11 designates a rotary drum (upper drum). Numerals 12a, 12b designate first and second video heads for recording reproduction respectively fixed using screws to grooves 11a, 11b formed on the upper drum 11 and spaced from each other by 180.degree.. Numeral 13 designates a piezoelectric element mounting base, and numeral 14 a disc-like piezoelectric element of sticking type having a circular hole at the center. Numerals 15a, 15b designate third and fourth video heads mounted on the free end of the piezoelectric element 14 and spaced from each other by 180.degree., and numerals 16a, 16b piezoelectric element pressure plates. In order to perform field reproduction without picture image fluttering, the rotary head is constituted by four heads. Among the four heads, the first and second video heads 12a, 12b are those for recording and normal reproduction and different from each other in azimuth angle, and residual third and fourth video heads 15a, 15b are those to be used exclusively for variable speed reproduction and displaced at the same azimuth angle. When prescribed voltage is applied to the piezoelectric element 14 by some means (not shown), the third and fourth video heads 15a, 15b are moved up and down in the rotational axis direction thereby the height H (refer to FIG. 12) can be adjusted.
If the position of the video head in the rotational axis direction (height H in FIG. 12) is varied, position of the scanning track on the magnetic tape by the video heads can be varied and the reproduction output can be controlled. Accordingly, in conventional method using a piezoelectric element, supply voltage applied to the piezoelectric element is controlled and height H is suitably varied so that the reproduction output in each mode becomes large.
The piezoelectric element has hysteresis .alpha. as shown in FIG. 13. Even if height H (refer to FIG. 12) is set at the initial stage, the initial height H cannot be restored sfter the current flowing Existence of the hysteresis .alpha. has no problem during the reproduction, because the piezoelectric element is used at current flowing state. However, during the recording, since the track is drawn in the state that the height H is shifted by .alpha., recording at a definite format cannot be performed. Consequently, in conventional method using a piezoelectric element, since first and second video heads 12a, 12b in fixed state must be installed for the recording, problem exists in that the number of heads becomes large.
Moreover, conventional method has problem in determining voltage to be supplied to the piezoelectric element, snd must be improved to obtain accurate recording pattern on the format.